CVD, PECVD and plasma etch processes (sometimes referred to herein as "semiconductor processes") use large amounts of a variety of process gases. The process gases used are often toxic, flammable and corrosive. Typically, only a small fraction of the process gases are consumed during the semiconductor process. As a result, the exhaust gas from the semiconductor process (which typically is a gas mixture) typically contains a substantial amount of unreacted process gases. To protect human health and the environment, as well as to comply with various emission restrictions, the exhaust gas from CVD, PECVD or other processes is treated to remove toxic and hazardous gas species before the exhaust gas is released to the atmosphere.
A number of methods are used to treat semiconductor process exhaust gas, each having a number of drawbacks. One common method is to combust the exhaust gas by burning the exhaust gas with fuel and air. However, fine particulates (dust) are produced as a byproduct of the combustion. Typically, the dust itself presents an environmental hazard and care must be exercised in collection of the dust. Frequently, the dust is collected by water scrubbing and subsequently the scrubbing water is treated to remove the dust particles. Further, the dust particles often clog various jets and burners used in the combustion system which causes the combustion system to fail.
Another conventional method of treating semiconductor process exhaust gas is by wet scrubbing. Wet scrubbing entails reacting the exhaust gas with water which causes water soluble gas species to dissolve in the water. The water is then treated to remove the dissolved material. However, water treatment units are expensive to operate and maintain. Further, as water use restrictions become increasingly common, wet scrubbing techniques become increasingly less attractive.
In Smith et al., U.S. Pat. No. 5,417,934, a dry exhaust gas treatment method is disclosed. The method comprises introducing the exhaust gas into a heated silicon stage which contains silicon particulates partially coated with copper and then into a heated calcium oxide stage which contains calcium oxide.
The exhaust gas reacts with the coated silicon and the calcium oxide in the first and second stages, respectively, to form involatile compounds which remain in the first and second stages. However, since the coated silicon and the calcium oxide occupy a substantial volume, the remaining volume available for the involatile compounds formed from the reacted exhaust gas is limited. This limits the operating life of the exhaust gas treatment unit. Further, the active chemical components, i.e. the coated silicon and calcium oxide, are relatively expensive and the exhaust gas treatment unit is also relatively expensive.
In Chiu, U.S. Pat. No. 4,735,633, an exhaust gas treatment method which uses a plasma extraction reactor is disclosed. The plasma extraction reactor comprises a pair of parallel, spaced-apart electrodes which are driven at radio frequency to induce a glow discharge in the waste species. The excited species are deposited directly on the electrode surface.
Of importance, the plasma extraction reactor is located in the sub-atmospheric pressure region (the foreline) between the semiconductor processing chamber in which the semiconductor processing occurs and the vacuum pump which removes the exhaust gas from the semiconductor processing chamber. The plasma extraction reactor must operate at sub-atmospheric pressure to effectively generate the plasma which causes the excited exhaust gas species to be deposited. During the required periodic servicing and replacement of the plasma extraction reactor, the foreline must be vented to atmospheric pressure. This necessitates removing the semiconductor processing chamber from service, which is undesirable. Further, the power supplies used to power the plasma extraction reactor, as well as the air to vacuum electrical interconnection required to supply the power to the plasma extraction reactor, are relatively expensive to purchase, maintain and operate.
The art needs an exhaust gas treatment apparatus which is cost effective and operates well in its intended environment. In particular, it is desirable to avoid generating dust particles and to avoid dissolving exhaust gas species in water. Furthermore, it is desirable that the apparatus have an extended lifetime and that the semiconductor processing not be interrupted during the servicing and replacement of the apparatus.